Free Compound Interest Calculator

For broadly diversified equity portfolios, the long-term US S&P 500 returns have averaged ~10% nominally and ~7% real (inflation-adjusted) over the last 50–100 years. Conservative planners use 6%; aggressive planners use 10%. Bond-heavy portfolios run 4–5%. Cash and high-yield savings: 2–5% depending on rate cycles. The Trinity Study and most retirement-research literature uses 6% real as a safe planning return.

Because real markets do. Mutual fund NAVs update daily, but for projection math monthly is the standard convention used by the SEC's investor.gov calculator, Vanguard, and Fidelity tools. The difference between monthly and continuous compounding on a 7% return is under 0.1 percentage points — invisible at typical investment horizons.

For headline numbers, nominal returns are fine — they're what your statement will literally show. For planning (e.g. "will I have enough to retire?") use real returns, because what matters is purchasing power, not the dollar count. Set the inflation field to your assumed long-term rate (2–3% is typical) to see your final balance in today's dollars.

No — projections are pre-tax. In a taxable brokerage account, your effective return is reduced by ~15–20% for long-term capital gains and qualified dividends taxes; in a Roth IRA / Roth 401(k) the projection is correct (no future tax); in a Traditional IRA / 401(k) you'll pay ordinary income tax on withdrawals. For tax-specific modeling, use the Roth vs Traditional IRA Calculator or the 401(k) Calculator.

Arithmetic average is just the mean of annual returns. Geometric (or CAGR) is the constant rate that would produce the same compounded result. Volatility drags down geometric returns relative to arithmetic. The S&P 500's arithmetic average is ~10–11%; its CAGR is closer to 9.5%. For multi-year compound projection, the geometric/CAGR number is what actually compounds. Enter the geometric estimate, not the arithmetic average.

A lot — usually 30–60% more in the final balance over a 30-year horizon. If you contribute 10% of a $60k salary at year 0 and that salary grows 3%/yr, by year 30 you're contributing 10% of $146k. Those later-year contributions are larger AND compound for fewer years, but the dollar magnitude compensates. The "contribution growth" field models this directly.

This calculator assumes a constant rate of return, which is mathematically clean but not how real markets work. The S&P 500 has 35% drawdowns roughly every 10 years on average. A 6% average return might be made up of years like −37%, +28%, +15%, −5%, +9% — the average is the same but the path matters during withdrawal (sequence-of-returns risk). For accumulation phase, the math is roughly accurate; for retirement spend-down, use Monte Carlo simulation tools.

No. All calculations run in your browser. Your starting balance and contribution amount never leave your device. The URL can encode your scenario so you can share it with a partner or financial planner — but the parameters live only in the link.